Stereochemical tests of cyclic 1,1-dialkyldiazene fragmentation reactions. Thermal decomposition of N-(cis-(and trans-)2,3-(and 2,5-)dimethylpyrrolidine)nitrenes

Abstract

The thermal decompositions of presumed 1,1-diazenes, N-(cis-(and trans-)2,5-dimethylpyrrolidine)nitrenes (30), generated from the base-induced decomposition of N-benzenesulfonamido-cis-(and trans-)2,5-dimethylpyrrolidines (28) in octane (and diglyme) at 120°C, afford propene, 1,2-dimethylcyclobutanes, and 1-hexene. The retention/inversion (r/i) ratios of closure products are higher than those found from the corresponding 1,2-diazenes in the gas phase at 306 and 439°C. The thermal decompositions of presumed 1, 1-diazenes, N-(cis-(and trans-)2,3-dimethylpyrrolidine)nitrenes (31 ), generated from the base-induced decomposition of N-bcnzenesulfonamido-cis-(and trans-)2,3-dimethylpyrrolidine (29) in octane (and diglyme) at ≤ 120°C, afford 2-butenes and 1,2-dimethylcyclobutanes. The stereospecificity in the 2-butanes and 1,2-dimethylcyclobutanes is high. Analysis of the data indicates a 51% direct cleavage pathway to trans-2-butene and ethylene and 49% 1,4-biradical pathway from the decomposition of N-(trans-(2,3-dimethylpyrrolidine)nitrenc (trans-31 ). Similarly, N-(cis(2,3-dimethylpyrrolidine)nitrene (cis-31) affords a 38% direct cleavage pathway to cis-2-butene and ethylene and 62% 1,4-biradical pathway. Whether this stereospecific cleavage reaction is a concerted cycloreversion or results from a diazenyl biradical precursor cannot be distinguished from the data. The relative rates of rotation, cleavage, and closure for 3-methyl-1,4-pentanediyl (35C and 35T) generated from the corresponding tetramethylene-1, 1-diazenes (31) in octane at ≤ 120°C were determined. From 35C, k(cleavage)/ k(closure) = 1.6, k(closure)/ k(rotation) = 4.9. From 35T, k(cleavage)/ k(closure) = 4.7 and k(closure) / k(rotation) = 3.1. Thermal decomposition of N-methanesulfonamido-cis-(and trans-)2,3-(and 2,5-)dimethylpyrrolidines (39 and 38, respectively) in the gas phase at 306 and 439°C affords hydrocarbon products consistent with the intermediacy and subsequent decomposition of I, 1-diazenes (30 and 31). The retention/inversion ratios in the closure products suggest that 1,4 biradicals generated from cyclic 1,1-dialkyldiazene decompositions in the gas phase at 306 and 439°C behave much like those from cyclic 1,2-diazene decompositions under similar conditions. 1,4 biradicals with secondary radical centers generated from the same 1,1-dia,zenes behave more stereospecifically in solution at lower temperatures (120°C).

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